1. Field of Invention
The present invention relates to a technique of detecting atmospheric temperature at high accuracy by using a hot wire airflow sensor. The present invention also relates to a technique of controlling an electric heater provided in a system of supplying a reducing agent or its precursor at high accuracy in response to atmospheric temperature in an exhaust emission purification apparatus which is provided for reducing nitrogen oxide (NOx) in exhaust emission from an engine by using the reducing agent.
2. Description of the Related Art
As a catalytic purification system for reducing NOx in engine exhaust emission, an exhaust emission purification apparatus described in Japanese Laid-Open (Kokai) Patent Application Publication No. 2000-27627 has been proposed. In such an exhaust emission purification apparatus, a reducing agent or its precursor according to an engine operating state is injected and supplied to exhaust emission at a position on upstream side of a reduction catalytic converter disposed in an engine exhaust emission system to cause a catalyst reduction reaction between NOx in the exhaust emission and the reducing agent, thereby purifying NOx to harmless components. Such an exhaust emission purification apparatus employs a technique of controlling an electric heater provided in a supply system of a reducing agent or its precursor on the basis of the atmospheric temperature detected by a temperature sensor so as to prevent the reducing agent or its precursor from being frozen and to promote defrosting the reducing agent or its precursor.
On the other hand, also in engine control, as described in Japanese Laid-Open (Kokai) Patent Application Publication No. 2005-207321, there is a technique of using atmospheric temperature as a control variable.
Although the atmospheric temperature is generally detected by a temperature sensor, in a different system having a hot wire airflow sensor, it is possible to detect intake air temperature due to the operating principle of detection of the sensor. Therefore, in order to satisfy various demands such as cost reduction, using the intake air temperature is employed as a practical technique in replacement of using the atmospheric temperature.
With the hot wire airflow sensor, it must be taken into consideration that since the hot wire airflow sensor is disposed near the engine, the sensor is actually susceptible to heat from the engine. In the hot wire airflow sensor, temperature compensation is performed by means of known a bridge circuit. Therefore, there is no substantial influence on intake air flow rate to be inherently detected. However, there is such an inconvenience that a temperature signal output by the hot wire airflow sensor is apt to be influenced by heat from the engine, and a difference appears between the temperature signal and an actual atmospheric temperature.
When the electric heater of the exhaust emission purification apparatus is controlled in response to such an atmospheric temperature, there is a possibility that various inconveniences occur such that the electric heater is operated although the reducing agent or its precursor is not frozen, or the electric heater is not operated although the reducing agent or its precursor is frozen. In engine control, there is such a possibility that an unexpected control is performed. It is also difficult to perform the engine control based on a temperature output signal coming from the hot wire airflow sensor.